Manufacture of zinc oxide



oci, 12, 1943.

H. M. CYR

MANUFACTURE OF ZINC OXIDE Filed July 26, 1939 2 Sheets-Sheet `l Filed July 26, 1939 Get. 12, 1943.

ATTORNEYS Patented Oct. 12, 1943 AMANUFACTURE OF ZINC OXIDE Howard M.v Cyr, Palmerton, Pa., assigner to The New Jersey Zine Company, New York, N. Y., a corporation of New Jersey Application July 26, 1939, Serial No. 286,557

Claims.

This invention relates to the production of zinc oxide of acicular habit by the combustion of zinc vapor in oxidizing gas and aims to provide improvements in process and apparatus to the end that the formation of such zinc oxide shall be facilitated and the size and shape of the particles of zinc oxide be controlled.

The invention contemplates the production of finely divided zinc oxide in which the great majority of particles are acicular in character, and, moreover, contemplates control of the condiv tions of particle formation so that substantially all of them are either well developed single needles or else complex twinned acicular crystals (such as threelings, fourlings, or even more complex forms), as desired. 'I'he invention also con- :all of the particles are line or coarse single needles or else fine or coarse complex acicular crystals.

is preferred to the nodular variety, because, under same conditions and with certain vehicles, acicular zinc oxide imparts improved durability to paint. In particular, such acicular zinc oxide decreases the tendency of certain paints to fail by checking and cracking.

There are two types of pyrometallurgical processes for the production of zinc oxide, the French process and the American process. In the French process zinc vapor is obtained by boiling metallic zinc and then burned, usually in undiluted form, in air to form zinc oxide. In the American process, zinc vapor is produced by reducing ore and then burned to zinc oxide while still results when zinc vapor and air are blasted together under high pressure, provided that combustion takes place in a zone having a temperature of about 1000 C. to 1500 C. and provided further that the air is sufficiently hot prior to admixture with the zinc vapor. If exceedingly small acicular particles are desired the zine vapor andair are mixed at high velocity, which may be attained by supplying the vapor or the air or both to the combustion zone under pres sure. In such case, the air should be at high temperature prior to contact with the zinc vapor and substantially at the temperature maintained in the combustion zone, which should'be between about 1000 C. and about 1500 C. but preferablyshould not exceed 140()u C. On the other hand, if zinc oxide having uniformly acicular particles of normal or relatively large di- For certain types 0f paint acicular zinc oxide mixed with products of combustion derived from the reduction fuel.

It has been proposed heretofore to form acicular zinc oxide in a modication of the American process in which zinc vapor diluted with carbonaceous gaseous combustion products is Iburned with a lazy flame and the resulting zinc oxide particles are maintained at a high temperatureintensity and even in the intense flame which mensions is desired,- the zinc vapor and the preheated air are mixed at substantially lower vclocity. Generally speaking, the lower the velocity of mixing, the larger is the size of the particles produced. In case the velocity of mixing the air and zinc vapor is low, air at lower temperatures can be used Without loss of acicularity; but air heated to atleast a moderate degree should be used, and it is preferable in this case also to preheat the air to 10001400tj C., i. e., substantially to the temperature desired in the combustion chamber. If the air is only moderately hot, say 500 C., before entering the combustion chamber, the amount of air should be controlled so that only a slight excess, say 10%, is used. Ii the air is very hot, a great excess can be used. By varying the velocity of mixing and by employing air with a mifcient degree of preheat, anyA desired particle size may be obtained within reasonable limits, and such results can be obtained Without sacrificing either acicularity of particles or uniformity of particle size.

In the practice of my invention, the zinc vapor introduced into the'combustion zone may be relatively undiluted (as in the French process) or it may be diluted with gaseous carbonaceous products, of the nature of producer gas (as in the American process).

In order to maintain proper thermal conditions in the combustion zone, it shouldbe enclosed within a chamber having' walls of heat insulating material, so as to avoid excessive dissipation of heat.

.In the practice of my` invention, it is desirable to remove all of the particles from the zone of formation as promptly as possible (thereby interrupting crystal growth) .and to limit recirculasince such recirculation tends to produce particles of undesirable platy form instead of acicular particles.

I have discovered that, in the manufacture of acicular zinc oxide by the process of my invention, it is possible to control not only the uniformity and size of the acicular particles, but also to control twin formation, i.` e., the tendency of acicular zinc oxide to form in ycomplex crystals, such as threelings and fourlings. Thus, I have discovered that if zinc vapor is burned in preheated air in a combustion zone having a pre dominant temperature ranging from 1000 C. and 1200" C. the particles tend to be in the form of complex twinned crystals of acicular habit, such as twolings, threelings and fourlings. However, if the predominant temperature of the combustion zone is raised to 12251500 C., the zinc oxide tends to form as single needles, although at temperatures in' excess of about 1400 C. some of the particles. are nodular and without visible -velocity for the formation of fine particles.

Moreover, if the temperature of the air is moderate, the use of air in large excess must be avoided, a factor that sets a further limit to velocity of mixing and hence to fine particle size. If. however, the air is preheated to approximately the temperature maintained in the combustion zone, say 1000 C, to 1400 C., the process is much more flexible with respect to the average size of particles which it can produce, since it is possible to maintain anydesired mixing velocity between zinc vapor and air without 'reducing the temperature in the combustion zone to a point below the temperature at which acicular oxide forms, i. e., below about 1000 C. With suchpreheated air, the zinc vapor can be burned in a most intense ame obtained by use of an air blast, and the zinc oxide formed will be substantially all acicular, albeit of fine particle size. If larger particle size is desired,I it can be obtained `A -by reducing the velocity with which the air and zinc vapors are mixed while maintaining other conditions the same.

The use of air preheated to a temperature in excess of 1000 C. aifords a further advantage, in that, as I have discovered. it is possible to regulate twinning and to produce at will either single acicular needles of zinc oxide or the more Acomplex forms, such as threelings or fourlings which result from twinning by regulating the temperature of the air over a range of about 1000 C. to about 1250 C. The temperature in the combustion zone tends to increase as the temperature of the preheated air is increased. Thus, if it is desired to produce zinc oxide particles of twinned form, with a predominant temperature of 1000 C. in the combustion zone, the

air should be preheated to 1000 C. If a product consisting substantially of single acicular crystals is desired, the air should be preheated in excess of 1200 C. and preferably to about 1250 C. v

As indicated hereinbefore, the conditions of gas ilow through the combustion zone preferably are so controlled that the particles of zince oxide fume formed during combustion are not recirculated through the zone by eddying currents to a great extent. The recirculation of zinc oxide fume through the combustion zone may be obviated by carrying out the combustion in a chamber having an outlet substantially equal to its cross section so as to avoid the large scale turbulence produced in the neighborhood of the small outlet. However, other mechanically equivalent means for avoiding large scale turbulencemay 'be employed.

In` the practice of the invention, conditions are so maintained that the combustion occurs completely in a well-defined zone of high temperature, i. e., in the zone maintained at the temperature of 1000 C. to say 1400 C., precautions being taken to avoid the formation of `zinc oxide fume outside this zone. Moreover, the gas flow in the combustion chamber is so controlled that zinc oxide fume when once formed is removed rapidly from the neighborhood of the combustion zone, thereby avoiding the formation of platy particles through recirculation in contact with oxidizing zinc.

Complete and thorough mixing of the hot air and zinc vapor is necessary for the production of acicular zinc oxide according to the process of my invention, since such mixing assures complete combustion of the zinc vapor within the hot combustion zone. 'I'he heated air for combustion preferably is supplied in the form of an annular or approximately annular jet surround- Ving Yor partly surrounding the stream of zinc vapor entering the combustion 'zone or in the form of elongated ribbon-like jets on each side of an entering ribbon-like stream or jet of zinc vapor. However, any method of supplying hot air that permits of thorough and controlled mixing of hot air and the zinc vapor (without the formation of large eddy currents that induce recirculation of zinc fume) may be ernployed. Thus, the hot air may be supplied in small Jets cutting through the zinc flame. Mechanical stirring may also be employed to control the speed of mixing of the hot air and the zinc vapor, provided that it is so conducted that large scale turbulence and consequent recirculation of zinc oxide particles in the combustion zone is avoided.

In short, mixing of zinc vapor and preheated air should be sulciently thorough to assure complete combustion in the zone of high temperature, but any small scale turbulence lincident to the thorough mixing should be spent in a relatively small space adjacent the point of initial contact of zinc vapor and air and not be transmitted to more remote parts of the combustion zone to such a degree as would induce substantial recirculation of zinc fume 1 through the zone.

, duction of acicular zinc oxide:

Fig. 2 is a section through the apparatus of Fig. 1 along the line II-1I;

Fig. 3 is a sectional elevation through a mod- A collecting means.

ifled type of apparatus of my invention for the production of acicular oxide;

Fig. 4 is a sectional elevation through still antype of single acicular needle of zinc oxide formed in the practice of my invention at high temperature and on which both ends of the crystal are formed by hexagonal pyramids;

Fig. 8 is an enlargement of a fourling of zinc oxide formed in the practice of my invention at temperatures below that at which single acicula needles are formed; and

Fig. 9 is an lenlargement of a more complex zinc oxide crystal formed in the practice of my invention at the lower temperature range.

Referring now to Figs. 1 and' 2, the apparatus comprises a zinc vapor combustion chamber Il which is a horizontally disposed elongated rectangular space of uniform cross section enclosed by Walls I2 of heat insulating material to facilitate the maintenance of high temperature. AZinc vapor is`introduced into an end of the chamber from any convenient source (such as a zinc boiler or a traveling grate furnace operated to reduce zinc ore) through an orifice I3 which is in the form of a narrow horizontally extending slot so that the zinc vapor enters the combustion chamber as a thin ribbon-like jet. Hot air is introduced into the chamber through apair of flues I4, I5 with elongatedslot-like cross section disposed, respectively, above and below the zinc vapor oriilce. The ues are inclined toward the axis oi' the zinc vapor orifice at such an angle that hot air introduced through the flues as thin ribbon-like jets on both sides of the ribbon of zinc vapor is rapidly and thoroughly mixed with the-zinc vapor so as to establish a short zone of intense combustion.

The hot air for combustion of the zinc vapor varies inversely as the intensity of mixing of the zinc and air, but should at all times be less than the volume of the chamber, soA that cornbustion is completed in the latter at a point well ahead of an outlet I1 from which the zinc fume is removed continuously and sucked to a bag house (notshown) or other conventional dust The combustion chamber is so adjusted with respect to form and size relative to the size of the combustion zone that large scale turbulence in the chamber is avoided, so as to prevent recirculation of the fume through the combustion zone. Thatl is to say, suiiicient space is provided around the combustion zone to enable the turbulence prevailing in the zinc ame on account of the intensive mixing of zinc vapor and air 'to spend itself without setting up large eddy currents that would tend to recirculate zinc oxide fume through the combustion zone.

A means for conserving and utilizing the heat yof combustion of the zinc vapor to preheat the air is illustrated in the apparatus of Fig. 3. In this apparatus, zinc vapor from any convenient source, such, for example, as a zinc retortor a traveling grate furnace is introduced into a combustion chamber 20 of rectangular and uniform cross section through a zinc vapor orice 2| of rectangular cross section and relatively wider than it is high. The combustion' chamber is formed of walls 22 of refractory heat conductive material, such,- for example, as silicon carbide and is surrounded by a passage or preheating chamber 23 through which air is passed. The air in passing through the preheating chamber is heated to the requisite temperature, say, in ex'- cess of 1000 C. and enters the combustion chamber through elongated openings 24, 25, respectively, above and below the zinc vapor orifice in much the same fashion as in the apparatus of Figs. 1 and` 2. The entire apparatus of Fig. 3 is surrounded by a wall 26 of heat insulating material. Acicular zinc oxide of desired particle size and shape results from the mixing and combustion of the ribbon of zinc vapor and the enveloping ribbons of preheated air, and is withdrawn immediately from a combustion zone 21,

that partially fills the combustion chamber, through a zinc oxide outlet 28. Asv in the case of the apparatus of Figs. v1 and 2, mixing of the preheated air and the zinc vapor should b-e thorough (and this is in large measure assured by bringing them together as thin ribbon-like jets passing in the samegeneral direction) but should be so conducted that no large scale turbulence is set up in the chamber to cause zinc fume to be recirculated through the combustion zone instead .of being -promptly and permanently exconvenient source, either in an undiluted condi' tion or mixed with carbonaceous gaseous com'- bustion products, is introduced into an upper end of the chamber through a conduit 32 which conveniently may be a slot in the front wall of the chamber, said slot being relatively long with respect to its height and extending across the chamber. Air for the combustion oi the zinc vapor is introduced at a lower portion of the opposite end of the chamber through a conduit or manifold 33. Conveniently, the air is blown into the chamber by means of a fan 33A or other draught producing device and by adjusting the speed of the fan the velocity of the entering air can be controlled to regulate intensity of combustion and hence shape of the particles. The zinc vapor in burning tends to confine itself to the upper portion of the chamber in a roughly outlined combustion zone 34. The zinc fume produced in the combustion zone is withdrawn at the upper opposite end of the combustion chamber through a conduit 35 from whence it passes through a fan or'other draught producing device (not shown) toa zinc oxide collecting device such as a bag house. The air introduced through the conduit' 33 tends to iiow along 'the sole of the chamber and to pass countercurrently in heat exchange rewhen it is deflected upwardly and mixes with tbe u vapor introduced through the conduit 32;

The operation should be so conducted that the air obtains a temperature in excess of 1000 C. before it comes in contact with the zinc vapor. This may be determined by means of a thermocouple 31 or otherheat indicating device dis- POSed in the front wall of the chamber along the upward path of the air. Likewise, a second thermocouple 38 should be disposed in the combustion zone to determine whether-or not the temperature up to this point is in the preferred range, i. e., between 1000 and 1400 C.

Operation of the apparatus illustrated in Figs. 4 and 5 has demonstrated that it is unnecessary to maintain any bailles or walls between the combustion zone and the preheating zone to avoid premature admixture of the air and zinc vapor and that the air stays in the lower portionof the chamber until it acquires an adequate degree of preheat.

If desired, in the apparatus of Figs. 1, 2 and 3, the preheating of the'air can be facilitated by i burning a relatively small `amount of combustible.

Sas therein :lust before thel air comes in contact with the zinc vapor. The amount of gas supplied must, of course, be insufcient to consume all of the oxygen in the air, since enough oxygen must be left to insure proper combustion of the zinc vapor.

In the operation of apparatus of any of the types illustrated, the air should be preheated substantially, say in excess of 500 C. and preferably should be heated to a temperature in excess of 1000 C. before it comes in contact with the zinc vapor. If the temperature of the preheated air is maintained in the range of 1000 C. to about 1200 C., and the temperature of the combustion zone does not exceed 1200 C. the oxide resulting from the process will be substantially all in the form of complex acicular crystalsi By raising the temperature of the preheated air to say 1200 C. or higher, a temperature in the combustion. zone exceeding 1225 C. can be readily attained, so as to produce a' zinc oxide consisting substantially of single needles.

I claim:

l. In the production oi zinc oxides of acicular habit, theimprovement which comprises introducing zinc vapor substantially free of zinc oxide fume and oxidizing gases into a .well dened combustion zone, burning the zinc vapor substantially completely in the well deiined combustion zone in preheated air while maintaining the zone in a temperature range of about 1000 C. to about 1200*? C., the air being preheated prior to contact with the zinc vapor to a temperature in the neighborhood of said range, and withdrawing the resulting zinc oxide particles of twinned acicular habit promptly and permanently from said zone as they are formed therein so that recirculation of the zinc oxide particles through the zone in contact with the burning zinc vapor is substantially avoided.

2. In the production of zinc oxide of acicular habit, the improvement which comprises introducing zinc vapor substantiallyrireogzinc oxide fume and oxidizing gases into a well deiined combustion zone, burning the zinc vapor substantially completely in the well deiined combustion zone in preheated air while maintaining the zone in a temperature range of about 1225 C. to 1500 C., the air being preheated prior to contact with the zinc vaporto a temperature of at least 1200 C., and withdrawing the resulting zinc oxide par'- ticles of single acicular'habit promptly'and permanently from said zone as they are formed therein so that recirculation of the zinc oxide particles through -the zone in contact with the burning zinc vapor is substantially avoided.

3. In the vproduction of zinc oxide of acicular habit, the improvement which comprises introducing zinc vapor substantially free of zinc oxide fume and oxidizing gases into a well dened combustionA zone, burning the zinc vapor substan- -tially completely in the well deiined combustion tact with the burning zinc vapor is substantially produced.

avoided. 4. In the production of zinc 'oxide of acic- Vlular habit, the improvement which comprises introducingl zinc vapor substantially free of zinc loxide fume and oxidizing "/gases into a well deiined combustion zone, burning the 'zinc vapor substantially completely to the well dened combustion zone in preheated air while maintaining the zone in a temperature range of about 1000 C. to about 1500 C., the air being preheated prior. to contact with the zinc vapor to a temperature in the neighborhood of that prevailing in the combustion zone, withdrawing the resulting particles Yof zinc oxide promptly and permanently from said zone as they are formed therein, so that recircu- 'lation of the zinc oxide particles'through the zone in contact with the burning zinc vapor is substantially avoided, and regulating the size of the resulting acicular particles of Zinc oxide by varying the velocity of admixture of the preheated air and the zinc vapor.

5. In the production of zinc oxide of acicular habit. the improvement which comprises introducing zinc vapor substantially free of zinc oxide fume and oxidizing gases into a well defined combustion zone, burning the zinc vapor substantially completely in the well dened combustion zone in preheated air while maintaining the zone in a temperature range of about 1000 C. to about 1500 C., the air being preheated prior to contact with the zinc vapor to a temperature in the neighborhood of that prevailing in the combustion zone and introduced into the zone at high velocity and at a pressure in excess of atmospheric pressure, and withdrawing the resulting particles of zinc oxide promptlyand permanently from said zone as they are formed therein so that recirculation of the zinc oxide particles through the zone in contact with the burning zinc vapor is substantially avoided, whereby acicular zinc oxide particles of small size are HOWARD M. 

